George a



(No Model.)

G. A. BROWN.

RHEOSTAT.

No. 515,280. Patented Feb. 20,1894.

mmemtoz:

UNITED STATES PATENT OFFICE.

GEORGE A. BROIVN, OE MUSKEGON, MICHIGAN, ASSIGNOR TO THE SI-IAV ELECTRICCRANE COMPANY, OF SAME PLACE.

RH EO STAT.

SPECIFICATION forming part of Letters Patent No. 515,280, dated February20, 1894.

Application filed August 19,1893. Serial No. 483,502- (No model.)

aZZ whom it may concern.-

Be it known that I, GEORGE A. BEowN, a citizen of the United States,residing at Muskegon, in the county of Muskegon and State of Michigan,have invented certain new and useful Improvements in Rheostats, of whichthe following is a specification.

My invention relates to rheostats and electrical resistance devicesgenerally, and consists essentially in a resistance card, block orsection, formed of metallic tape or ribbon, folded back and forth uponitself in ziz-zag form, the folds being insulated one from another by anatural scale or oxide existing or formed upon the tape or ribbon.

In the drawing, the figure is a perspective view of a resistance card orsection constructed in accordance with my invention.

The purpose of my invention is to produce a cheap, durable and efiicientresistance, which may be made of varying capacity within wide limits,and which shall occupy comparatively little space. To this end I employmetallic ribbon or tape, preferably hoop or band iron,--and I fold thesame back and forth upon itself, making the length of the foldsdependent somewhat upon the nature of the current for which it isdesigned, though they should in all cases be sufficiently short topreclude short-circuiting from fold to fold.

The tendency of the current to jump from fold to fold instead of passingthrough the entire length of all the folds, varies with the differenceof potential at any point, between the folds. The greater the differenceof potential or drop, the greater the tendency to jump or short circuit,and the drop is of course dependent upon the strength of the current andthe resistance of the fold. Forthese reasons, and in order not to makethe resistance of any fold unduly high, the folds should be made ofcomparatively short length; but this is a matter of judgment, variableaccording to working conditions.

Metallic tape or ribbon, hoop or band iron, in its commercial state,bears a film or coating of mill scale, which, in the case of iron, isferrous oxide,a quite poor conductor of electricity,-or a comparativelygood insulator. Various other metals and alloys of metals when rolledinto the form of strips, bands or ribbons, possess or acquire a coatingof oxide, and metallic oxides generally are poor conductors ofelectricity, or quite good insulators; hence, while iron is preferredfor various reasons, my invention is not restricted to its use, butcomprehends any metal or alloy possessing such scale, film or coating ofmetallic oxide.

The natural mill scale of iron is ferrous oxide; but this, if exposed tomoisture, even to that present in the atmosphere, is changed to ferricoxide, which otters higher electrical resistance than does ferrousoxide.

It may be advisable in some cases to subject the metal to the action ofcorroding agents to cause the formation of a heavier or more perfectcoating of oxide. This may best be done after the metal is folded, butwhile the folds are still sufficiently free or open to permit theentrance of the corroding agent between them, because the mill scale isliable to be cracked and broken more or lessin making the short bends.By thus restoring the oxide at proximate bends, the current will beprevented from short circuiting from one to another.

Referring again to the drawing, A indicates a resistance card, block orsection, composed of a folded tape, ribbon or band a, and renderedcompact and firm by suitable binding bands I), advisably of wire, and insuch case suitably insulated from the strip a. Such insulation maybeeffected by interposing strips 0 of mica, asbestus or otherincombustible insulating material between the band a and the wrapping orbinding wire b, as indicated in Fig. 1, or in any usual and equivalentmanner. The ends of band or strip amay be connected in any convenientmanner with contact blocks or binding screws, and two or more cards maybe connected in series, with suitable contact blocks for each, after themanner of rheostats now in general use.

Electrical resistance devices have before been made of wire, and ofmetallic strips bent in ziz-zag form, and I do not broadly claim suchconstruction; but so far as I am aware, no one has ever before bound afolded metallic strip into a firm and compact body or card,

ICC

1. A resistance card or body composed of a strip or band of metal foldedback and forth upon itse1f,the folds being separated only by the scaleor oxide formed upon their faces.

2. A resistance card or body composed of a metallic strip or band afolded back and forth upon itself and having metallic oxide upon itssurface, and bands 19, passing about the folded strip and serving tobind and retain it in compact form.

3. A resistance card or body consisting of a strip of iron, with themill scale or oxide upon it, folded back and forth upon itself in acompact form.

In witness whereof I hereunto set my hand in the presence of twowitnesses.

GEO. A. BROWN.

Witnesses:

A. J. SHAW, DAVID D. ERWIN.

